52 FERTILISERS CONTAINING NITROGEN [chap. 



these zeolites and exchange bases to an extent depend- 

 ing upon the relative masses of the two bodies ; hence 

 nitrate of soda acts on the clay in the soil and brings a 

 little potash into solution. To such an extent does this 

 action take place that in practice a dressing of nitrate 

 of soda on any but the lightest soils will dispense with 

 the necessity of a specific potash manuring, even for 

 potash-loving crops. 



This is well illustrated in the Rothamsted experi- 

 ments (see Table X.) upon mangolds, if we compare the 

 yields on the plots receiving equivalent amounts of 

 nitrogen as nitrate of soda, sulphate of ammonia, and 

 rape cake, both with and without potash. The table 

 refers to the season of 1900, the twenty-fifth year of 

 that series of experiments, when it might be sup- 

 posed the potash in the soils of the plots receiving no 

 potash in the manure must have become thoroughly 

 exhausted : — 



Table X.— Effect of Soda in Nitrate of Soda, Mangolds, 

 Rothamsted, 1900. 



' 



The plots receiving potash all give about the same 

 yield, whatever the source of nitrogen ; but on plots 5, 

 without potash, the yield is only maintained on the 

 nitrate of soda plot ; on the other two the plant is 

 neither supplied with potash by the manure, nor is the 

 soil forced to yield some of its stored-up potash as it is 

 by the nitrate of soda, whereupon the yield declines by 



